A couple of suggestions for you..
1) Papaya , a shop on the 5th floor of CentralWorld , they also have a branch at MBK , both Bangkok.
2) www.watchbyladda.com , I have bought Rolex watches from them twice , they also have a bricks and mortar shop , but they are only interested in buying good condition , relatively new pieces with full box & papers..
Hope this has been helpful..